Dipole strengths in 11Be from electroproduction of charged pions.

Abstract

The dipole resonances in the light nucleus $^{11}\mathrm{Be}$ were studied measuring the energy spectra and angular distributions of charged pions in the spin-flip charge-exchange reaction $^{11}\mathrm{B}$(e,${\mathrm{\ensuremath{\pi}}}^{+}$${)}^{11}$Be at ${\mathit{E}}_{\mathit{e}}$=200 MeV. The ground state and eight excited states below ${\mathit{E}}_{\mathit{x}}$=18 MeV in $^{11}\mathrm{Be}$ were observed. The angular distributions were analyzed within the framework of the distorted-wave impulse approximation. A considerable amount of the deduced E1 strengths is distributed over excitation energies much lower than 18 MeV in $^{11}\mathrm{Be}$. The energy distributions of the E1 strengths of transitions to the s- and d-shell states were compared with the results of theoretical calculations with the effect of loosely bound neutron states.